Process for forming an aramid copolymer

ABSTRACT

The invention concerns processes for forming a polymer comprising residues of 2-(4-amino phenyl)-5(6) amino phenyl benzimidazole (DAPBI), paraphenylene diamine (PPD), and terephthaloyl dichloride, comprising the steps of: (a) forming a slurry of DAPBI in a solvent system comprising an organic solvent and an inorganic salt; (b) adding terephthaloyl dichloride to the slurry terephthaloyl dichloride in the amount of up to one-half mole for every mole of DAPBI in the slurry; (c) agitating the slurry to react the DAPBI and terephthaloyl dichloride to form an oligomeric solution; (d) adding PPD to the oligomeric solution and agitating until substantially all of the PPD is dissolved, (e) adding terephthaloyl dichloride in an amount of greater than one mole for every mole of PPD in the solution to form a prepolymer solution; and (e) agitating the prepolymer solution to form a polymer.

TECHNICAL FIELD

The present application concerns methods of producing aramid polymersderived from 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI),para-phenylenediamine (PPD) and terephthaloyl dichloride (TCl) that arecapable of forming fibers having superior physical properties.

BACKGROUND

Fibers derived from 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI),para-phenylenediamine (PPD) and terephthaloyl dichloride (TCl or T, alsocommonly referred to as terephthaloyl chloride) are known in the art.Such copolymers are the basis for a high strength fibers manufactured inRussia, for example, under the trade names Armos® and Rusar®. See,Russian Patent Application No. 2,045,586.

The two amines on DAPBI are very different in reactivity and positionalfactors. The amine shown to the right in the below structure (the azoleamine) is an order of magnitude more reactive than the amine to the leftin the structure (the benzyl amine).

As a result, DAPBI/PPD copolymers prepared by conventional apolymerization process in NMP/CaCl₂ solvent system tend to have nocontrol over the position of the monomer components. It is believed afactor in creating higher strength fibers from DAPBI/PPD copolymer isthe arrangement of the comonomers along the polymer chain. Inparticular, controlling the distribution of PPD and DAPBI comonomers isbelieved to help prevent the formation of crystal solvates in sulfuricacid solution and help the alignment of polymer chains during the heattreatment of fibers made from the copolymer, resulting in fibers withbetter mechanical properties.

SUMMARY

In some aspects, the invention concerns processes for forming a polymercomprising residues of 2-(4-amino phenyl)-5 (6) amino phenylbenzimidazole (DAPBI), PPD, and terephthaloyl dichloride, comprising thesteps of: a) forming a slurry of DAPBI in a solvent system comprising anorganic solvent and an inorganic salt; b) adding terephthaloyldichloride to the slurry in an amount of up to one-half mole for everymole of DAPBI in the slurry; c) agitating the slurry to react the DAPBIand terephthaloyl dichloride to form an oligomeric solution; d) addingPPD to the oligomeric solution and agitating until substantially all ofthe PPD is dissolved, e) adding terephthaloyl dichloride in an amount ofgreater than one mole for every mole of PPD in the solution to form aprepolymer solution; and e) agitating the prepolymer solution to form apolymer. The process can further comprise the step of dissolving thepolymer in a solvent comprising sulfuric chloride to form a solutionsuitable for spinning fibers.

In some processes, the invention concerns processes for forming apolymer comprising residues of 2-(4-amino phenyl)-5 (6) amino phenylbenzimidazole (DAPBI), paraphenylene diamine (PPD), and terephthaloyldichloride, comprising the steps of: (a) forming a solution of oligomershaving amine end groups from two moles of DAPBI and one mole ofterephthaloyl dichloride in a solvent system comprising an organicsolvent and an inorganic salt; (b) adding PPD to the solution ofoligomers; and (c) adding additional terephthaloyl dichloride to form apolymer. In some embodiments, the amount of terephthaloyl dichlorideadded in step c) is adequate to achieve stoichiometric balance based onthe amount of diamines in the solution.

Preferred organic solvents include N-methyl-2-pyrrolidone (NMP) anddimethylacetamide (DMAC). Preferred inorganic salts include LiCl andCaCl₂. In certain embodiments, the solvent system is NMP/CaCl₂.

The invention can involve the further step of isolating the polymer.Some embodiments of the invention involve a step comprising the step ofcomminuting the polymer. Polymer, comminuted or not comminuted, can betreated with one or more washing steps, neutralizing steps, or both.

In some aspects, the invention additionally concerns the step ofdissolving the polymer in a solvent comprising sulfuric acid to form asolution suitable for spinning fibers. The polymer to be dissolvedincludes the isolated polymer that may or may not have been washedand/or neutralized and polymer may or may not have been comminuted.Alternately, an organic solvent (optionally containing an inorganicsalt) can be used to form the solution suitable for spinning fibers.While any suitable solvent can be used to dissolve the polymer, in someembodiments the solvent comprises N-methyl-2-pyrrolidone (NMP) ordimethylacetamide (DMAC) and an inorganic salt to form a solutionsuitable for spinning fibers. Examples of suitable inorganic salts areLiCl and CaCl₂.

In some preferred embodiments, the molar ratio of DAPBI to phenylenediamine is in the range of from 0.25 to 4.0. For some processes, theamount of the slurry that is DAPBI in step (a) is in the range of from 1to 10 weight %. In certain processes, the amount of oligomeric solutionthat is PPD in step b) is in the range of from 0.5 to 5.5 weight %. Incertain embodiments, the additional terephthaloyl dichloride in step c)is an amount that is in the range of from 100 to 400 mole percentrelative to the amount of PPD added in step b).

For some processes, the NMP/CaCl₂ solvent has a CaCl₂ weight percent inthe range of from 0.3 to 10%.

In yet another aspect, the invention concerns a polymer comprising2-(4-amino phenyl)-5 (6) amino phenyl benzimidazole (DAPBI), PPD, andterephthaloyl dichloride, the polymer having a IPC peak block ratio of1.52 to 1.56, preferably 1.53-1.55, and an inherent viscosity of greaterthan 2 dl/g. Some preferred polymers have an inherent viscosity of 4dl/g or greater. In some embodiments, the polymer is capable of beingdissolved in a solvent system comprising (i) N-methyl-2-pyrrolidone(NMP) or dimethylacetamide (DMAC) and (ii) an inorganic salt; whereinthe polymer is capable of being redissolved in the solvent system afterthe polymer has been removed from the solvent system.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description, isfurther understood when read in conjunction with the appended drawings.For the purpose of illustrating the invention, there is shown in thedrawings exemplary embodiments of the invention; however, the inventionis not limited to the specific methods, compositions, and devicesdisclosed. In the drawings:

FIG. 1 presents IPC results of the examples versus that of DAPBI-Thomopolymer.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention can be understood more readily by reference to thefollowing detailed description taken in connection with the accompanyingfigures and examples, that form a part of this disclosure. It is to beunderstood that this invention is not limited to the specific devices,methods, conditions or parameters described and/or shown herein, andthat the terminology used herein is for the purpose of describingparticular embodiments by way of example only and is not intended to belimiting of the claimed invention.

In some aspects, the invention concerns processes for forming a polymercomprising residues of 2-(4-amino phenyl)-5 (6) amino phenylbenzimidazole (DAPBI), PPD, and terephthaloyl dichloride, comprising thesteps of: a) forming a slurry of DAPBI in a solvent system comprising anorganic solvent and an inorganic salt; b) adding terephthaloyldichloride to the slurry in an amount of up to one-half mole for everymole of DAPBI in the slurry; c) agitating the slurry to react the DAPBIand terephthaloyl dichloride to form an oligomeric solution; d) addingPPD to the oligomeric solution and agitating until substantially all ofthe PPD is dissolved, e) adding terephthaloyl dichloride in an amount ofgreater than one mole for every mole of PPD in the solution to form aprepolymer solution; and e) agitating the prepolymer solution to form apolymer. The process can further comprise the step of dissolving thepolymer in a solvent comprising sulfuric chloride to form a solutionsuitable for spinning fibers.

The instant process allows two moles of DAPBI to react with one mole ofterephthaloyl dichloride first. It is believed the faster reactingamines (the azole amines) react with both acid chlorides on theterephthaloyl dichloride (TCl) molecule creating a distribution ofoligomers that preferentially comprise “trimers” ending with slowerreacting amines (the benzyl amines) as terminal groups. In theseresulting trimers, the two end amine groups have equivalent reactivity.

To this trimer, paraphenylene diamine (PPD) is added and dissolved, andadditional terephthaloyl dichloride (TCl) monomer is added to makeDAPBI/PPD-T copolymers with controlled head-to-tail placement of DAPBIunits. As an added benefit, the resulting polymer is soluble in organicsolvent system such as DMAc/LiCl as well as sulfuric acid.

Vessels useful for producing polymers, and temperatures and otherconditions useful in producing polymers, include, for example, detailsdisclosed in such patents as U.S. Pat. No. 3,063,966 to Kwolek et al.;U.S. Pat. No. 3,600,350 to Kwolek; U.S. Pat. No. 4,018,735 to Nakagawaet al.; and U.S. Pat. No. 5,646,234 to Jung et al.

In some embodiments, the organic solvent is N-methyl-2-pyrrolidone (NMP)or dimethylacetamide (DMAC). Suitable inorganic salts include LiCl andCaCl₂. In one preferred embodiment, the solvent system is NMP/CaCl₂. Incertain embodiments, the NMP/CaCl₂ solvent has a CaCl₂ weight percent inthe range of from 1 to 10%. It should be noted that the solubility ofCaCl₂ in NMP is about 8%. As such when more than 8% CaCl₂ is used, someundissolved CaCl₂ is present in the solvent system. The solvents andsalts can be obtained from commercial sources and, if desired, purifiedby methods known to those skilled in the art.

In some processes, the molar ratio of DAPBI to paraphenylene diamine isin the range of from 0.25 to 4.0. This ratio is equivalent to aDAPBI/PPD ratio of 20/80 to 80/20. In certain processes, the amount ofthe slurry that is DAPBI in step (a) is in the range of from 0.5 to 10weight %.

For some processes, the amount of paraphenylene diamine added in step(d) is in the range of from 0.2 to 6.0 weight %, based on all thecomponents in the oligomeric solution.

In some embodiments, step d) or e) or both steps d) and e) is/areperformed under agitation. The polymer can be isolated in someembodiments. The isolated polymer can be comminuted a desired particlesize to assist in processing and storage. The polymer can be treatedwith one or more washing steps, neutralizing steps, or both. Thesewashing and/or neutralizing steps can be performed before or aftercomminuting the polymer. Equipment suitable for use in agitation of thereaction mixtures, washing and neutralization steps, and comminuting thepolymer is known to those skilled in the art.

Molecular weights of polymers are typically monitored by, and correlatedto, one or more dilute solution viscosity measurements. Accordingly,dilute solution measurements of the relative viscosity (“V_(rel)” or“η_(rel)” or “n_(rel)”) and inherent viscosity (“V_(inh),” or “η_(inh)”or “n_(inh)”) are typically used for monitoring polymer molecularweight. The relative and inherent viscosities of dilute polymersolutions are related according to the expression

V _(inh)=ln(V _(rel))/C,

where ln is the natural logarithm function and C is the concentration ofthe polymer solution. V_(rel) is a unitless ratio, thus V_(inh) isexpressed in units of inverse concentration, typically as deciliters pergram (“dl/g”).

Neutralization of the polymer can occur in one or more steps bycontacting the polymer with a base. Suitable bases include NaOH; KOH;Na₂CO₃; NaHCO₃; NH₄OH; Ca(OH)₂; K₂CO₃; KHCO₃; or trialkylamines,preferably tributylamine; other amines; or mixtures thereof. In oneembodiment, the base is water soluble. In some preferred examples theneutralization solution is an aqueous solution of base.

The polymer can also be washed with water independent of or prior toand/or after the neutralization step.

In some aspects, the invention additionally concerns the step ofdissolving the polymer in a solvent comprising sulfuric acid to form asolution suitable for spinning fibers (also referred to as a “spindope”). The polymer to be dissolved includes the isolated polymer thatmay or may not have been washed and/or neutralized and polymer may ormay not have been comminuted. While any suitable solvent can be used todissolve the polymer, in some embodiments the solvent comprisesN-methyl-2-pyrrolidone (NMP) or dimethylacetamide (DMAC) and aninorganic salt to form a solution suitable for spinning fibers. Thedissolved polymer can be spun into fibers by conventional techniquesknown to those skilled in the art.

The spin dope containing the copolymer described herein can be spun intodope filaments using any number of processes; however, wet spinning and“air-gap” spinning are the best known. The general arrangement of thespinnerets and baths for these spinning processes is well known in theart, with the figures in U.S. Pat. Nos. 3,227,793; 3,414,645; 3,767,756;and 5,667,743 being illustrative of such spinning processes for highstrength polymers. In “air-gap” spinning the spinneret typicallyextrudes the fiber first into a gas, such as air and is a preferredmethod for forming filaments

As used herein, the terms filaments and fibers are used interchangeably.

The fiber can be contacted with one or more washing baths or cabinets.Washes can be accomplished by immersing the fiber into a bath or byspraying the fiber with the aqueous solution. Washing cabinets typicallycomprise an enclosed cabinet containing one or more rolls that the yarntravels around a number of times, and across, prior to exiting thecabinet. As the yarn travels around the roll it is sprayed with awashing fluid. The washing fluid is continuously collected in the bottomof the cabinet and drained therefrom.

The temperature of the washing fluid(s) is preferably greater than 30°C. The washing fluid can also be applied in vapor form (steam), but ismore conveniently used in liquid form. Preferably, a number of washingbaths or cabinets are used. The residence time of the yarn in any onewashing bath or cabinet will depend on the desired concentration ofresidual sulfur in the yarn. In a continuous process, the duration ofthe entire washing process in the preferred multiple washing bath(s)and/or cabinet(s) is preferably no greater than about 10 minutes, morepreferably greater than about 5 seconds. In some embodiments theduration of the entire washing process is 20 seconds or more; in someembodiments the entire washing is accomplished in 400 seconds or less.In a batch process, the duration of the entire washing process can be onthe order of hours, as much as 12 to 24 hours or more.

If needed, neutralization of the acid (such as sulfuric acid solvent) inthe yarn can occur in a bath or cabinet. In some embodiments, theneutralization bath or cabinet can follow one or more washing baths orcabinets. Washes can be accomplished by immersing the fiber into a bathor by spraying the fiber with the aqueous solution. Neutralization canoccur in one bath or cabinet or in multiple baths or cabinets. In someembodiments, preferred bases for the neutralization of sulfuric acidimpurity include NaOH; KOH; Na₂CO₃; NaHCO₃; NH₄OH; Ca(OH)₂; K₂CO₃;KHCO₃; or trialkylamines, preferably tributylamine; other amines; ormixtures thereof. In one embodiment, the base is water soluble. In somepreferred examples the neutralization solution is an aqueous solutioncontaining 0.01 to 1.25 mols of base per liter, preferably 0.01 to 0.5mols of base per liter. The amount of cation is also dependent on thetime and temperature of exposure to the base and the washing method. Insome preferred embodiments, the base is NaOH or Ca(OH)₂.

After treating the fiber with base, the process optionally can includethe step of contacting the yarn with a washing solution containing wateror an acid to remove all or substantially all excess base. This washingsolution can be applied in one or more washing baths or cabinets.

After washing and neutralization, the fiber or yarn can be dried in adryer to remove water and other liquids. One or more dryers can be used.In certain embodiments, the dryer can be an oven that uses heated air todry the fibers. In other embodiments, heated rolls can be used to heatthe fibers. The fiber is heated in the dryer to a temperature of atleast about 20° C. but less than about 100° C. until the moisturecontent of the fiber is 20 weight percent of the fiber or less. In someembodiments the fiber is heated to 85° C. or less. In some embodimentsthe fiber is heated under those conditions until the moisture content ofthe fiber is 14 weight percent of the fiber or less. The inventors havediscovered that low temperature drying is a preferred route to improvedfiber strength. Specifically, the inventors have found that the bestfiber strength properties are achieved when the first drying step (i.e.heated roll, heated atmosphere as in an oven, etc.) experienced by thenever-dried yarn is conducted at gentle temperatures not normally usedin continuous processes used to dry high strength fibers on commercialscale. It is believed that the copolymer fiber has more affinity towater than PPD-T homopolymer; this affinity slows the diffusion rate ofwater out of the polymer during drying and consequently if thenever-dried yarn is directly exposed to typical high dryingtemperatures, generally used to create a large thermal driving force andreduce drying time, irreparable damage to the fiber occurs resulting inlower fiber strength. In some embodiments, the fiber is heated at leastto about 30° C.; in some embodiments the fiber is heated at least toabout 40° C.

The dryer residence time is less than ten minutes and is preferably lessthan 180 seconds. The dryer can be provided with a nitrogen or othernon-reactive atmosphere. The drying step typically is performed atatmospheric pressure. If desired, however, the step can be performedunder reduced pressure. In one embodiment, the filaments are dried undera tension of at least 0.1 gpd, preferably a tension of 2 gpd or greater.

The invention also concerns a polymer powder comprising residues of2-(4-amino phenyl)-5 (6) amino phenyl benzimidazole (DAPBI),paraphenylene diamine, and terephthaloyl dichloride, capable of beingdissolved in N-methyl-2-pyrrolidone (NMP) or dimethylacetamide (DMAC)and an inorganic salt. In other words, the polymer is capable of beingredissolved in the solvent system after the polymer has been removed orisolated from the solvent system. It should be noted that polymers madeby traditional processes, once isolated, do not have the capability ofbeing redissolved in a solvent system of NMP or DMAc and an inorganicsalt but require a solvent such as sulfuric acid to achieve a spinningsolution. In some embodiments, the polymer powder has an inherentviscosity of greater than 2 dl/g. In some preferred embodiments, thepolymer has an inherent viscosity of 4 dl/g or greater.

In yet another aspect, the invention concerns a polymer comprising2-(4-amino phenyl)-5 (6) amino phenyl benzimidazole (DAPBI), PPD, andterephthaloyl dichloride, the polymer having a IPC peak block ratio(Relative Elution Time of sample/DAPBI-T homopolymer) of 1.52 to 1.56,preferably 1.53-1.55, and an inherent viscosity of greater than 2 dl/g.In some preferred embodiments, the polymer has an inherent viscosity of4 dl/g or greater.

DEFINITIONS

As used herein, the term “residue” of a chemical species refers to themoiety that is the resulting product of the chemical species in aparticular reaction scheme or subsequent formulation or chemicalproduct, regardless of whether the moiety is actually obtained from thechemical species. Thus, a copolymer comprising residues of paraphenylenediamine refers to a copolymer having one or more units of the formula:

Similarly, a copolymer comprising residues of DAPBI contains one or moreunits of the structure:

A copolymer having residues of terephthaloyl dichloride contains one ormore units of the formula:

Dashed lines indicate bond positions.

The term “polymer,” as used herein, means a polymeric compound preparedby polymerizing monomers, whether of the same or a different type. Theterm “copolymer” (which refers to polymers prepared from two differentmonomers), the term “terpolymer” (which refers to polymers prepared fromthree different types of monomers), and the term “quadpolymer (whichrefers to polymers having four different types of monomers) are includedin the definition of polymer.

The term “powder” when referring to polymer means particles of thecopolymer having neither fibrous qualities like fiber or pulp, norfibrous film-like qualities like fibrids. Individual particles tend tobe fibril-free, have a random shape, and an effective particle diameterof 840 micrometers or less. U.S. Pat. Nos. 5,474,842 & 5,811,042 areillustrative.

As used herein, “stoichiometric amount” means the amount of a componenttheoretically needed to react with all of the reactive groups of asecond component. For example, “stoichiometric amount” refers to themoles of terephthalic dichloride needed to react with substantially allof the amine groups of the amine component (paraphenylene diamine andDAPBI). It is understood by those skilled in the art that the term“stoichiometric amount” refers to a range of amounts that are typicallywithin 10% of the theoretical amount. For example, the stoichiometricamount of terephthalic dichloride used in a polymerization reaction canbe 90-110% of the amount of terephthalic acid theoretically needed toreact with all of the paraphenylene diamine and DPABI amine groups.

The “fiber” refers to a relatively flexible, macroscopically homogeneousbody having a high ratio of length to width across its cross-sectionalarea perpendicular to its length. In preferred embodiments, the fiber isupon inspection essentially solid in cross-section, having few randomvoids or open areas that would be considered defects in the fibers.

The term “organic solvent” is understood herein to include a singlecomponent organic solvent or a mixture of two or more organic solvents.In some embodiments, the organic solvent is dimethylformaaide,dimethylacetamide (DMAC), N-methyl-2-pyrrolidone (NMP), ordimethylsulfoxide. In some preferred embodiments, the organic solvent isN-methyl-2-pyrrolidone or dimethylacetamide.

The term “inorganic salt” refers to a single inorganic salt or to amixture of two or more inorganic salts. In some embodiments, theinorganic salt is sufficiently soluble in the solvent and liberates anion of a halogen atom having an atomic number in the solvent. In someembodiments, the preferred inorganic salt is KCl, ZnCl₂, LiCl or CaCl₂.In certain preferred embodiments, the inorganic salt is LiCl or CaCl₂.

By “never-dried” it is meant the moisture content of the fiber is atleast 75 weight percent of the fiber.

As used in the specification including the appended claims, the singularforms “a,” “an,” and “the” include the plural, and reference to aparticular numerical value includes at least that particular value,unless the context clearly dictates otherwise. When a range of values isexpressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. Allranges are inclusive and combinable. When any variable occurs more thanone time in any constituent or in any formula, its definition in eachoccurrence is independent of its definition at every other occurrence.Combinations of substituents and/or variables are permissible only ifsuch combinations result in stable compounds.

Test Methods

Inherent viscosity can be determined using a solution in which a polymeris dissolved in a concentrated sulfuric acid with a concentration of 96wt % at a polymer concentration (C) of 0.5 g/dl and at a temperature of25° C. Inherent viscosity is then calculated as ln (t_(poly)/t_(solv))/Cwhere t_(poly) is the drop time for the polymer solution and t_(solv) isthe drop time of the pure solvent.

Interaction Polymer Chromatography (IPC) method was used to analyze themicrostructure of p-aramid copolymers. The chromatographic separationswere performed on Alliance 2695™ Separation Module with column heater at60° C. from Waters Technologies (Milford, Mass., USA). The moduleprovides a low-pressure quaternary gradient pumping system with lagvolume 0.6 ml to the column outlet, online solvent degassing andautomatic sample injection from 2-ml vials. Waters UV/Vis 487™photometer at 320 nm wavelength was used as an online detector. Twocomponents of a mobile phase used were water (component A) andN,N′-dimethylformamide (DMAc) with 4% lithium chloride (LiCl) (componentB). 20 minutes linear gradient from 70% B to 100% B at flow rate 0.5ml/min was used for separation. Silica NovaPak™ 150×3.9 mm with 60 Apore size from Waters was used as a stationary phase. Each sample wasdissolved in DMAc with 4% LiCl at 120° C. with moderate agitation for 12hours at concentration 0.2 mg/ml, and injected using 10 ml injectionloop.

Empower™ version 2 software module from Waters was used for dataacquisition and reduction. The IPC peak block ratio is then determinedby running a data set including a control of DAPBI-T homopolymer and thecopolymer to be evaluated. The FIGURE is an typical illustration of thepeaks for the homopolymer and copolymers being evaluated, with thevertical arrows indicating the peak values. The IPC peak block ratio isthen calculated by the following formula:

IPC peak block ratio=Minutes to peak for copolymer/Minutes to peak forhomopolymer.

The invention is illustrated by the following examples, which are notintended to be limiting in nature.

EXAMPLES

NMP, DMAC, LiCl, CaCl₂, DAPBI, PPD and TCl were obtained from commercialsources.

Example 1

This example illustrates the preparation of DAPBI/PPD-T copolymers withcontrolled “Head-Tail” DAPBI Placement. DAPBI[5(6)-amino-2-(p-aminophenyl)benzimidazole] has two amine groups havinggrossly different reactivity, so-called asymmetric diamine. Amine groupattached to the benzene ring with fused imidazole ring react an order ofmagnitude faster than the amine group on the benzene ring alone atopposite side of the molecule. Head and tail in this example stand forthe faster/slower reacting amines, respectively; that is, the “Head” isthe benzyl amine and the “Tail” is the azole amine.

Polymer Preparation:

To 1 liter reaction kettle equipped with basket stirrer, nitrogeninlet/outlet, 83.75 grams of NMP/CaCl₂ premix (8.3 wt % (weight ofsalt/total weight of salt plus solvent)), 163.30 g of NMP(N-methyl-2-pyrrolidone], and 12.288 g (0.055 moles) of DAPBI was addedand stirred for 10 minutes. At this point DAPBI was not completelydissolved in the solvent system. The content was stirred in ice-waterbath to cool the mixture below 10° C. 5.562 grams (0.027 moles) of TCl(terephthaloyl dichloride) was added all at once and stirred for 5minutes. At this point, the solution became clear as DAPBI reacted withTCl. The ice water bath was removed, and 2.539 grams (0.023 moles) ofPPD (p-phenylene diamine) was added and stirred until all PPD wasdissolved in the solution. 10.340 grams (0.051 moles) of TCl was addedall at once and stirred. The solution became very viscous and gelled inabout 4 minutes. The highly viscous reaction mixture was then stirredfor another 25 minutes. The resulting polymer was transferred to Waring®Blender and ground to small particles and washed several times withwater to remove solvent (NMP/CaCl₂) and excess HCl generated by thereaction. Then the polymer was neutralized with sodium bicarbonate andfinally washed with water a few times to get neutral polymer. Thepolymer was transferred into a tray and dried at 120° C. overnight in avacuum oven with a nitrogen sweep. The inherent viscosity of the polymerwas 8.15.

Solubility of the Polymer in Organic Solvent (DMAc/LiCl (4 wt % (Weightof Salt/Total Weight of Salt Plus Solvent)):

0.125 grams of above polymer was added to a solvent of 25 ml of(DMAc/LiCl, 4 wt % (weight of salt/total weight of salt plus solvent))in a glass vial, and stirred in a shaker at room temperature. Thepolymer dissolved completely and provided a clear solution.

Comparison Example

This example illustrates the preparation of DAPBI/PPD-T copolymer inNMP/CaCl₂ solvent by a conventional way of adding TCl to the solution ofboth DAPBI and PPD in the solvent.

83.71 g of NMP/CaCl₂ premix (8.3 wt % (weight of salt/total weight ofsalt plus solvent)), 163.32 g of NMP, and 2.538 g [0.023 moles] of PPDwere added to a 1-liter reaction kettle equipped with basket stirrer andnitrogen inlet/outlet. The mixture was stirred at room temperature untilall PPD was dissolved completely in the solvent, then 12.282 g (0.055moles) of DAPBI was added and stirred for 15 more minutes at roomtemperature. The solution looked milky due to the undissolved DAPBI. Themixture was cooled to below 10° C. by placing in an ice water bath andstirred for 15 minutes. A first portion of 5.573 g (0.027 moles) of TClwas then added and stirred for 5 minutes. The ice-water bath was removedand a second portion of 10.351 g of TCl was then added all at once andstirred. The solution became very viscous and gelled within 4 minutesand the stirring continued for another 25 minutes. Highly viscous lumpsof polymer were transferred to a Waring® blender and ground to smallparticles and washed several times to remove solvent (NMP/CaCl₂) andexcess HCl generated by the reaction. Then the polymer was neutralizedwith sodium bicarbonate and finally washed with water a few times to getneutral polymer. The polymer was transferred into a tray and dried at120° C. over night in vacuum oven with nitrogen sweep. The measuredpolymer inherent viscosity was 5.47 dl/g.

Solubility of the Polymer in Organic Solvent(DMAc/LiCl(4 wt % Weight ofSalt/Total Weight of Salt Plus Solvent)

0.125 grams of the polymer of the Comparative Example was added to 25 mlof (DMAc/LiCl, 4 wt % (weight of salt/total weight of salt plussolvent)) in a glass vial, and stirred in the shaker at roomtemperature. The polymer did not dissolve at all and there was noindication of swelling.

Determination of IPC Values

IPC peak block ratios were determined for the polymer of Example 1 andthe Comparison Example by the test method described above. Results arepresented below.

Peak Value Peak Ratio Homopolymer (Control) 18.09 min NA Example 1 27.94min 1.54 Comparison 27.09 min 1.50

What is claimed:
 1. A process for forming a polymer comprising residuesof 2-(4-amino phenyl)-5 (6) amino phenyl benzimidazole (DAPBI), PPD, andterephthaloyl dichloride, comprising the steps of: a) forming a slurryof DAPBI in a solvent system comprising an organic solvent and aninorganic salt; b) adding terephthaloyl dichloride to the slurry in anamount of up to one-half mole for every mole of DAPBI in the slurry; c)agitating the slurry to react the DAPBI and terephthaloyl dichloride toform an oligomeric solution; d) adding PPD to the oligomeric solutionand agitating until substantially all of the PPD is dissolved, e) addingterephthaloyl dichloride in an amount of greater than one mole for everymole of PPD in the solution to form a prepolymer solution; and e)agitating the prepolymer solution to form a polymer.
 2. The process ofclaim 1, wherein said solvent system is NMP/CaCl₂.
 3. The process ofclaim 2, further comprising isolating said polymer.
 4. The process ofclaim 3, further comprising the step of comminuting the polymer.
 5. Theprocess of claim, further comprising treating the polymer with one ormore washing and/or neutralizing steps.
 6. The process of claim 5,further comprising the step of dissolving the polymer in a solventcomprising sulfuric chloride to form a solution suitable for spinningfibers.
 7. The process of claim 6, wherein the DAPBI and phenylenediamine are present in a molar ratio in the range of from 0.25 to 4.